The present invention relates to a method for processing fibers. More specifically, the present invention relates to a process for preparing biodegradable fibers useful as a fiber product for agriculture, civil engineering and fishing industry, and biodegradable fiber obtained by the process.
There is a growing social awareness that a large amount of plastics are discarded into natural environment to cause environmental destruction. This recognition has promoted development of biodegradable plastics which can be decomposed and reduced into carbon dioxide and water in a natural environment. By preparation methods, presently known biodegradable plastics can be classified into chemically synthesized materials such as polylactic acid and polybutylene succinate, natural product containing materials such as starch, cellulose, and blended materials of these and other degradable plastics, and polyester produced by microorganisms such as poly-3-hydroxybutylates and poly-3-hydroxyalkanoates.
The polyester produced by microorganisms is a storage substance accumulated in the body of microorganisms. This is a polymer substance which serves as an energy source for microorganisms in starvation.
In natural world, there are many microorganisms which degrade polyester produced by microorganisms. The polyester produced by microorganisms has excellent feature that it can be degraded fast by biological methods in natural environment such as soil, river, lakes, seawater, activated sludge or compost. Another excellent feature of the polyester produced by microorganisms is that it has thermoplasticity just like general plastics and can be processed into various forms according to usual processing methods of plastics. In this respect, the polyester produced by microorganisms is a polymer far more useful than natural products such as starch and cellulose.
Polyhydroxyalkanoate, one of the typical polyesters produced by microorganisms, is an aliphatic polyester biosynthesized by an internal enzyme of microorganisms, which has limited number of structures because of the specificity of the enzyme. The most known polyester produced by microorganisms is poly-3-hydroxybutylate (hereinafter referred to as P(3HB)) represented by the following structural formula: 
However, the polymer is hard and brittle. Though commercial production of the polymer was once undertaken, the polymer failed to become widespread because its properties were unsatisfactory. In order to overcome this defect, ICI. has tried some culture methods and succeeded in the copolymerization of the material (EP 52459 and EP 69497). This is a method of fermentation synthesis of a random copolymer (hereinafter referred to as P(3HB-CO-3HV) comprising two monomer units, that is, 3-hydroxybutylate (3HB) and 3-hydroxyvalylate (3HV) in the coexistence of glucose and propionic acid, which are inherently used as carbon source in incubating microorganisms which biosynthesize P(3HB). The copolymer has become commercially available by the trade name xe2x80x9cBIOPOLxe2x80x9d. The structure of the copolymer is as follows: 
On the other hand, microorganisms are isolated from nature, which biosynthesize a random copolymer (hereinafter referred to as P(3HB-CO-3HH) comprising two monomer units, that is, 3-hydroxybutylate (3HB) and 3-hydroxyhexanoate (3HH)(Japanese Patent No. 2777757). The structure of the copolymer is as follows: 
Properties of the above copolymers P(3HB), P(3HB-CO-3HV) and P(3HB-CO-3HH) are made obvious in the research paper by Doi et al, xe2x80x9cMacromolecules, Vol. 28, No. 14, 1995xe2x80x9d. For example, P(3HB) is hard and brittle as mentioned above, and it is a homopolymer which has a homogeneous properties. As to P(3HB-CO-3HV), no significant change is observed in crystallinity though its properties depend on the composition of 3HV, and there is no significant change in elasticity even if the composition of 3HV is increased. That is, it does not happen that elongation is well over 100%. This is because 3HB and 3HV have structural difference in only one methylene group in the side chain. On the other hand, crystallinity of P(3HB-CO-3HH) decreases rapidly and significant change of properties is observed when the composition of 3HH is increased. This is because 3HB and 3HH have structural difference in two methylene groups in the side chain. Comparison of properties of P(3HB-CO-3HV) and P(3HB-CO-3HH) is shown in Tables 1 and 2.
In this way, it is obvious that properties of polyesters produced by microorganisms can be changed when copolymerized and that the structure of units also results in significant difference in properties.
Polyesters produced by microorganisms are one of the aliphatic polyesters having thermoplasticity, which can be molded just like other general plastics according to various processing methods. For example, P(3HB-CO-3HV) is processed in various ways and sold by the trade name xe2x80x9cBIOPOLxe2x80x9d Methods of processing P(3HB-CO-3HH) are disclosed in Japanese Unexamined Patent Publication Nos. 508424/1997, 508426/1997 and 128920/1998. However, as to the process for spinning P(3HB-CO-3HH) fibers, Japanese Unexamined Patent Publication No. 508424/1997 only discloses a general fiber spinning method and an example to prepare staple fibers having a length of 1.3 to 15 cm by jetting the fibers into rapid air stream from the nozzle of the extruder. And there is no report on the process for preparing P(3HB-CO-3HH) drawn filament.
Japanese Examined Patent Publication No. 63056/1990 discloses a method of spinning polyester produced by microorganisms which comprises holding P(3HB) or P(3HB-CO-3HV) at temperature ranging from the melting point xe2x88x9240xc2x0 C. to the melting point, holding the same at not more than 100xc2x0 C. for 1 to 120 seconds, and drawing the same 1.2-fold. In addition, Japanese Examined Patent Publication No. 63055/1990 discloses a method which comprises cooling melt-molded articles in water bath to carry out partial crystallization, and drawing drawable partially-crystallized undrawn filament by utilizing the peripheral speed ratio of rollers in a temperature range of the maximum crystallization temperature xe2x88x9230xc2x0 C. to the maximum crystallization temperature +30xc2x0 C. Furthermore, there are reports on methods further comprising pre-heating step before drawing, and heating step after drawing so that the rapid cooling step and the drawing step are carried out separately (Japanese Patent Publication Nos. 2815260, 2883809 and 2892964). However, such spinning methods had a problem that solidification does not proceed when spinning of P(3HB-CO-3HH) is intended, and end breakage or adhesion of fibers to the water bath guide is caused, resulting in unsuccessful fiber spinning.
Referring now to FIG. 1, a melt extrudate 2 is extruded from a die 1 of the melt extruder and cooled in a water bath 3 through guides 4 and 5 to obtain fixed monofilament 6 in which crystallizaiton is partially progressed. Subsequently, the fixed monofilament 6 is contacted with a heated pin 8 through a taken up roll 7 and heated on a heated plate 9. Thereafter, the filament is rolled on a reel 10 through the taken up roll 7.
An object of the present invention is to provide melt-extrusion conditions and drawing process which achieve stable fiber spinning of polyester produced by microorganisms which conventionally had problems in spinning stability, and smooth spinning of P(3HB-CO-3HH) as well as P(3HB) and P(3HB-CO-3HV) to obtain filament having particular properties.
As a result of intensive studies on the cause of unsuccessful fiber spinning of P(3HB-CO-3HH) in conventional methods, it became apparent that P(3HB-CO-3HH) did not have high crystallinity as P(3HB) or P(3HB-CO-3HV). It also became clear that it was necessary to carry out drawing by controlling melt viscosity of filament, solidifying the filament surface rapidly, and carrying out partial crystallization of the polymer rapidly, because crystallization speed of P(3HB-CO-3HH) was lower than that of P(3HB) or P(3HB-CO-3HV). Consequently, steps has been found for stable fiber spinning of P(3HB-CO-3HH), in which the surface of melted filament extruded from the melt extruder is cooled to at most the glass transition point to solidify the surface and the filament is prevented from blocking to carry out partial crystallization rapidly at not less than the glass transition point. It has also been found that the obtained pre-drawn filament can be drawn further, is capable of inhibiting growth of sphaerite and has sufficient elasticity and strength.
That is, the present invention relates to a process for producing a biodegradable fiber which comprises steps of: preparing a melted filament by extruding a thermoplastic polymer comprising polyhydroxyalkanoate from a melt-extruder; rapidly cooling the filament to at most the glass transition point of the thermoplastic polymer; passing the filament through a hot water bath adjusted to a water temperature of at least the glass transition point; and drawing the same.
It is preferable that the rapidly cooling step is a step in which the filament is rapidly cooled by passing the filament through a cooling cylinder provided below an outlet of the melt extruder to lower the temperature of at least the surface of the melted filament to at most the glass transition point.
It is preferable that the polyhydroxyalkanoate is a copolymer containing at least 3-hydroxybutylate and 3-hydroxyhexanoate.
It is preferable that the water temperature of the hot water bath is from at most the glass transition point to at least the maximum crystallization temperature +20xc2x0 C. of the thermoplastic polymer, and that the filament is partially crystallized by passing the filament through the water bath adjusted to the temperature.
The temperature of the drawing step is preferably in the range of the maximum crystallization temperature of the thermoplastic polymer xe2x88x9220xc2x0 C. to the maximum crystallization temperature of the thermoplastic polymer +20xc2x0 C.
It is preferable that the drawing step is carried out at a drawing ratio of 2 to 8-fold.
It is preferable that further drawing is carried out for 1 to 3 times under the same condition at a drawing ratio of 1.2 to 4-fold after the drawing step.
It is preferable that the process further comprises a heat treating step after the drawing step.
It is preferable that the heat treating step is carried out in a temperature range of the maximum crystallization temperature of the thermoplastic polymer xe2x88x9220xc2x0 C. to the maximum crystallization temperature of the thermoplastic polymer +2020  C.
The present invention also relates to biodegradable fiber obtained by the process.